This invention relates to protective relays and more particularly to microprocessor-based protective relays.
There are a number of different types of protective relays. Generally, the main types of protective relays include directional protective relays, differential protective relays and distance protective relays.
A directional protective relay utilizes a directional protection scheme to determine whether a fault has occurred in a forward direction, i.e., in the direction of a protected zone, or in a reverse direction, i.e., in the direction of a non-protected zone. A directional protection scheme generally includes an operating element and a direction or polarizing element. The operating element compares an incoming current (or voltage) with a pre-defined current (or voltage) setting. In the case of an overcurrent directional protection scheme, the operating element generally makes a trip decision based on whether current exceeds a pre-defined current tolerance. The polarizing element determines the direction of a fault. The polarizing element may use torque equations to determine fault direction; positive torque results from forward faults and negative torque from reverse faults. The torque equations may use zero sequence voltage or current, or a negative sequence voltage for polarizing. Alternately, the polarizing element may use negative-sequence impedance. Tripping only occurs when the operating element indicates a fault condition and the polarizing element indicates that the fault condition is in the protected zone. Examples of directional protection schemes are disclosed in U.S. Pat. No. 4,825,327 to Alexander et al. and U.S. Pat. No. 4,453,191 to Wilkinson, both of which are hereby incorporated by reference.
Differential protective relays are frequently used for the protection of generators, transformers, and station buses. A differential protective relay operates on the principal of balancing or comparing secondary currents of current transformers located at input and output terminals of protected equipment. The differential protective relay is disposed between the current transformers such that no differential current flows through the protective relay under normal conditions because the secondary currents through the current transformers are balanced. When an external fault occurs current flow increases at both the input and output terminals of the protected equipment, but the balance between these currents is maintained. Therefore, the protective relay does not operate for the external fault condition. When a fault occurs in the protected equipment, the current flow on one side of the protected equipment is reversed, thus upsetting the normal current balance at the protective relay. The unbalanced condition causes a differential current to flow through the protective relay, and a differential protection scheme in the protective relay operates to trip the appropriate circuit breaker. An example of a differential protective relay is disclosed in U.S. Pat. No. 4,502,086 to Ebisaka, which is hereby incorporated by reference.
A distance protective relay utilizes the principle that an electrical line has an impedance that is proportional to the length of the line. A distance protective relay measures the impedance of a line and compares it to a pre-defined impedance setting proportional to the full length of the line. If the impedance is less than the pre-defined impedance, a fault is determined to have occurred. An impedance or RX diagram is often used to describe the characteristics of a distance relay. An RX diagram is a plot of R (abscissa) versus X (ordinate) and shows the characteristics of a relay in terms of the ratio of voltage to current and the angle between them. In an RX diagram, the numerical value of the ratio of voltage to current is shown as the length of a radius vector and the phase angle between voltage and current determines the position of the vector. In this manner, the operating characteristic of a relay is shown as a circle on an RX diagram, with the tripping of the relay occurring within the circle.
There are a number of different types of distance protection schemes that may be utilized by a distance protective relay. These schemes include impedance, reactance, mho, offset mho and quadrilateral schemes. An impedance scheme does not take into account the phase angle between the voltage and the current applied to it. For this reason, the operating characteristic of an impedance scheme, as represented in an RX diagram, is a circle with its center at the origin. A reactance scheme measures only the reactive component of impedance. A mho scheme or starting unit is essentially a reactance scheme with a directional element. The operating characteristic of a mho scheme, as represented in an RX diagram, is a circle which passes through the origin. The operating characteristic of an offset mho scheme, as represented in an RX diagram, is a circle that is shifted and includes the origin, thus providing better protection for close-in faults. A quadrilateral scheme combines directional and reactance characteristics with two resistive reach control characteristics.
Examples of distance protection schemes are shown in U.S. Pat. No. 5,956,220 to Novosel et al. and U.S. Pat. No. 5,140,492 to Schweitzer, III, both of which are hereby incorporated by reference.
In addition to the foregoing general classes of protective relays, there are protective relays that utilize delta filters to detect faults in power circuits. In such a relay, a delta filter receives a voltage or current time-varying waveform from a power circuit and subtracts the waveform present at a selected interval of time prior to the present time from the present time waveform. This operation is typically accomplished using a delay function of the filter. The selected interval of time is equal to a selected integral multiple of the time-varying voltage/current waveform time period. Conventionally, the delay is one power circuit cycle. Under normal conditions, when there is no disturbance or fault event, the output of the delta filter will be zero. Then, when an event or fault in the power circuit occurs, a change in the current or voltage waveform will occur and the delta filter will have a non-zero output. The magnitude of the change is indicative of the significance of the actual change in the power circuit, as represented by the voltage and/or current values.
Examples of protective relays that utilize delta filters include U.S. Pat. No. 4,409,636 to Brandt et al., U.S. Pat. No. 5,783,946 to Yang and U.S. Pat. No. 6,417,791 to Benmouyai et al., all of which are hereby incorporated by reference. In the Brandt et al. patent, a phase is determined to have a fault if its signal has an amplitude that is greater than a certain percentage (such as 40-60%) of the greatest amplitude of the phase signals. Such a method of determining a phase fault is rather rigid and does not permit different fault determination criteria for one phase, two phase and three phase faults.
It would therefore be desirable, to provide a protective relay with delta filters that has a more flexible fault determination that permits different fault determination criteria for one phase, two phase and three phase faults. The present invention is directed to such a protective relay.